With the original rear gearing, a truck doing 65mph will have EVERYTHING wound right out... and may break.
Lower the rear gearing and your engine and tranny will be turning at a reasonable speed, transferring reasonable torque and power to the wheel. Everything will be happy (except maybe your brakes... and the guy stopped in front of you!). If you can find a full working center section from the right vehicle, it's a simple swap and your torque tube and hinged axle will work just fine.

Current fleet (subject to change w/o notice)
\'49 GMC 3/4-Ton , 60 Austin Healey Frog-eye Sprite (some assembly required), 2011 Dodge Avenger, 2015 Jeep Cherokee. No, they don't all run.
My other ride is a (B737)no, (B767)no, A320.... Update... was Embraer E190, now Boeing B787.

Knowledge is a wonderful thing, but ignorance means you don't know you can't do something.